The invention relates generally to apparatus for the carbonization of a continuous length of organic fibrous material. More specifically, the invention relates to a vertical pyrolysis furnace for use in carbonization apparatus wherein a continuous length of stabilized fibrous material moves downwardly through the pyrolysis furnace and gaseous by-products of the pyrolysis process move vertically upwardly through the furnace, under the influence of free convection and, if desired, an imposed pressure gradient, to exit the furnace for subsequent treatment and disposal.
In the past, it has been known to graphitize continuous lengths of organic fibrous material by the sequential steps of stabilization, pyrolysis and graphitization. Typically, stabilization of an organic fibrous material renders the same non-burning when subjected to an ordinary match flame and capable of undergoing carbonization without substantial loss of its original fibrous configuration. The stabilized fibrous material is subsequently passed through a pyrolysis operation (i.e., a carbonization operation) wherein elements present therein other than carbon are substantially expelled. Subsequent to the pyrolysis operation, a graphitization step may occur in which the X-ray diffraction pattern of the fiber is changed from a generally amorphous carbon pattern to a predominately graphitic carbon pattern. Significant decomposition during pyrolysis occurs at temperatures in the range of 400.degree. to 1000.degree. C. with appreciable devolatilization occurring up to about 1500.degree. C.; whereas the graphitization step occurs in the neighborhood of 2000.degree. C. or higher.
Most known apparatus for the production of graphitized fibers includes a horizontally disposed pyrolysis oven. Such horizontally disposed furnaces, however, are subject to numerous disadvantages. For example, suitable tension must be applied to a continuous length of fibrous material passing through a horizontal pyrolysis oven in order to compensate for the classical catenary effect. In addition, mechanical devices are often provided to specifically accommodate for a catenary tendency of a continuous length of fibrous material. Another disadvantage of horizontal pyrolysis ovens concerns the presence of an increased concentration of gaseous by-products. Since the presence of such gaseous by-products has a tendency to inhibit the efficiency with which additional gaseous by-products are thermally driven from the fibrous material, increased concentrations thereof are counter productive in the pyrolysis environment.
Other difficulties with horizontal ovens relate to the ability to provide a controlled temperature gradient axially therealong. A gradually increasing temperature gradient is desirable since increased amounts of gaseous by-products are released as the temperature level of the fibrous material is increased to the vicinity of 1000.degree. C.
With horizontal ovens, the condensation of gaseous by-products on cooler portions of the furnace creates an undesirable build-up of residue which can snag fibrous material passing therethrough or otherwise interfere with efficient operations. Moreover, the removal and treatment of toxic gaseous by-products present further difficulties in horizontal ovens since there is no preferred location to which the gases migrate for efficient collection and exhaustion. The foregoing lack of effective off-gas isolation to a non-deleterious temperature regime and lack of minimization of off-gas concentration severely limits the attainable mass throughput of fibrous material and, consequently, limits the process economics.
During pyrolysis, concentrations of gaseous off-products have a tendency to attack and corrode a continuous length of fibrous material from which they evolved unless they are promptly removed. This corrosion is aggravated by the presence of a hot environment and can cause significant strength reductions. With horizontal furnaces, it is therefore necessary to continuously force a purge gas through the furnace to sweep the off-products away. Since the purge gas thus needs pumping, frequently a turbulent flow regime exists which has a tendency in itself to accelerate the corrosion as well as to sharply limit process economics.
As a result of the catenary shape assumed by a continuous length of fibrous material as it traverses a horizontal pyrolysis furnace, the lateral dimensions of the furnace passageway must be sized to accommodate variable vertical sagging distances which result with changes in tension on the continuous length of fibrous material.
With the current emphasis on efficient utilization of available space, it will also be apparent that horizontal furnaces are generally undesirable. Moreover, since cost economies result from increased fibrous material line speed and since the line speed may be increased when the longitudinal dimension of a pyrolysis furnace increases, the value of an alternative to horizontal furnaces is even more clearly apparent.
We also note that horizontal furnaces are difficult to string up with a continuous length of fibrous material because limber fibrous material does not transmit compressive forces required to push it through a horizontal passageway.
Another problematic aspect of horizontal furnaces concerns the simultaneous processing of several continuous lengths of fibrous material. More specifically, interference between the continuous lengths themselves as well as with the gaseous off-products are undesirable.
A vertically disposed pyrolysis furnace having a pair of parallel upright walls each of which is provided with heating elements and between which a supply of fibrous carbonaceous material is passed vertically upwardly is known in the art. See, for example, U.S. Pat. No. 3,849,332 issued to Bailey et al. on Nov. 19, 1974. Such a device, however, does not permit a multizone heating system that establishes a means for a temperature gradient. In addition, there is no means for preventing contamination of an environment with the toxic gaseous off-products. Moreover, the vertical oven of the Bailey et al. apparatus does not permit the establishment of a concentration gradient for gaseous off-products which decreases as fibrous material advances through the furnace and which sweeps itself toward the exhaust.
The foregoing deficiencies of Bailey et al are further compounded by the furnace enclosure which necessitates semicontinuous operation and by the floor exhaust port. The exhaust port location of Bailey et al is not conducive to efficient removal of the off-gases by virtue of their relative buoyancy. Moreover, the off-gas concentration would be such that primarily low temperature operation would be dictated in order to prevent the off-gas from attacking the product being treated.
In one attempt to prevent the condensation and deposition of product gases, such as sodium, on the walls of a pyrolysis furnace, a counter flowing inert gaseous atmosphere is provided in the relatively hot portion of a pyrolysis furnace. A suitable exhausting means is provided with its separate heater to prevent exhaust gases from condensing thereon during their extraction or removal from the hot zone of the furnace. See, for example, British Pat. No. 1,284,399 issued to Courtalds Ltd., Aug. 9, 1972. Such an apparatus does not, however, provide an inherent means of preventing the condensation of gaseous products in the inlet portion of the pyrolysis device where wall surfaces are relatively cool and condensation tends to become a serious problem. In addition, no axially spaced heating zones are provided that enable a temperature gradient to be maintained during continuous operation.
While condensation of metals, such as sodium treated by the Courtalds Ltd. patent, is a known problem in carbonization at temperatures greater than 1000.degree. C., it has not been found to be an important problem for temperatures less than 1200.degree. C.